A distributed source is defined notably as a source which is propagated through a continuum of diffusers.
The invention makes it possible notably to locate, in angles and/or in azimuth, one or more distributed radio frequency sources. The object is, for example, to determine the incidence of the centers of the diffusion cones and their widths.
The goniometry is produced either in one dimension, 1D, where the incidences are parameterized by the azimuth, or in two dimensions, 2D, where the incidence depends on both azimuth and elevation parameters.
It applies, for example, for decorrelated or partially decorrelated coherent signals originating from diffusers.
FIG. 1 diagrammatically represents the example of diffusion of the wave from cell phone M through a layer of snow NG, for example to the receivers Ci of the reception system of an airplane A. The cone, called diffusion cone, has a certain width and an average incidence. The snow particles NG act as diffusers.
In the field of antenna processing, a multiple-antenna system receives one or more radiocommunication transmitters. The antenna processing therefore uses the signals originating from multiple sensors. In an electromagnetic context, the sensors are antennas. FIG. 2 shows how any antenna processing system consists of an array 1 with several antennas 2 (or individual sensors) receiving the multiple paths from multiple radiofrequency transmitters 3, 4, from different incidence angles and an antenna processing device 5. The term “source” is defined as a multiple path from a transmitter. The antennas of the array receive the sources with a phase and an amplitude dependent on their incidence angle and on the positioning of the antennas. The incidence angles can be parameterized, either in 1D azimuth-wise θm, or in 2D, azimuth-wise θm and elevation-wise Δm. FIG. 3 shows that a goniometry is obtained in 1D when the waves from the transmitters are propagated in one and the same plane and a 2D goniometry must be applied in other cases. This plane P can be that of the array of antennas where the elevation angle is zero.
The main objective of the antenna processing techniques is to exploit the space diversity, namely, the use of the spatial position of the antennas of the array to make better use of the incidence and distance divergences of the sources. More particularly, the objective of the goniometry or the locating of radiofrequency sources is to estimate the incidence angles of the transmitters from an array of antennas.
Conventionally, the goniometry algorithms such as MUSIC described, for example, in reference [1] (the list of references is appended) assume that each transmitter is propagated according to a discrete number of sources to the listening receivers. The wave is propagated either with a direct path or along a discrete number of multiple paths. In FIG. 2, the first transmitter referenced 3 is propagated along two paths of incidences θ11 and θ12 and the second transmitter referenced 4 along a direct path of incidence θ2. To estimate the incidences of all of these discrete sources, their number must be strictly less than the number of sensors. For sources that have diffusion cones of non-zero width, the goniometry methods described in document [1] are degraded because of the inadequacy of the signal model.
References [2] [3] [4] propose solutions for the goniometry of distributed sources. However, the proposed goniometry algorithms are in azimuth only: 1D. Also, the time signals of the diffusers originating from one and the same cone are considered to be either coherent in references [2] and [3], or incoherent in references [3] [4]. Physically, the signals of the diffusers are coherent when they are not temporally shifted and have no Doppler shift. Conversely, these signals are incoherent when they are strongly shifted in time or when they have a significant Doppler shift. The time shift of the diffusers depends on the length of the path that the waves follow through the diffusers and the Doppler depends on the speed of movement of the transmitter or of the receivers. These comments show how references [2] [3] [4] do not handle the more common intermediate case of diffusers with partially correlated signals. Also, the algorithms [2] [4] strongly depend on an “a priori” concerning the probability density of the diffusion cones angle-wise. It is then sufficient for these densities to be slightly different from the “a priori” for the algorithms [2] [4] no longer to be suitable.